(b).

Author	Title	Year	Country	Objective	Sample size	BDNF genetic variant	Genotypic frequency (Val/Val)	Laboratorial method	Results	p value (case vs. control)	Odds ratio
Froud et al. [21]	The relationship between dietary quality, serum brain-derived neurotrophic factor (BDNF) level, and the Val66met polymorphism in predicting depression	2019	Australia	Investigate the complex relationship between food quality, BDNF protein serum levels, and depression. Moreover, evaluate whether the BNDF Val66Met polymorphism influences this relationship.	n = 187 F = 136 (72.7%) M = 51 (27.3%)	rs6265	66.9% (n = 81)	Sequencing	Both higher serum BDNF (p < 0.001) and lower dietary quality scores (p = 0.037) were significantly detected in depression.	0.864	—
Ai et al. [22]	Plasma brain-derived neurotrophic factor (BDNF) concentration and the BDNF Val66Met polymorphism in suicide: a prospective study in patients with depressive disorder	2019	China	Investigate the correlation between BDNF protein concentration and its (196C>A) polymorphism and SSRI response in a Chinese MDD population.	n = 125 F = 76 (60.8%) M = 49 (39.2%)	rs6265	24% (n = 30)	PCR-RFLP	BDNF protein concentrations were significantly lower in patients who attempted or suffered from suicidal ideation. BDNF might be beneficial as a biomarker in antidepressant treatment response.	0.194	—
Kitzlerová et al. [23]	Interactions among polymorphisms of susceptibility loci for Alzheimer's disease or depressive disorder	2018	Czech Republic	Examine the contribution between multiple functional polymorphism interactions in the risk of MDD and Alzheimer's.	n = 68 F = 53 (77.9%) M = 15 (22.1%)	rs6265	77.9% (n = 53)	PCR-RFLP, per Chou et al. [24]	The G/A (Val/Met) genotype frequency is significantly lower in MDD, indicating its protective effects. Furthermore, the combination of 2 to 5 different gene polymorphisms has a significant cumulative effect on the severity of MDD.	0.79	0.44 (G/A)